This invention relates to an automatic welding apparatus for performing downward butt or horizontal fillet welding operations on the exterior and/or interior sheet-to-sheet joint portions of long, box-like or tubular workpieces having narrow entry apertures and more particularly to an arrangement wherein a movable welding torch is carried by a positioning assembly mounted on a pair of turn frames rotatable around a horizontal axis.
It is well known in the prior art to movably mount a welding torch by utilizing a positioning assembly adapted to control the spatial orientation of the tool with respect to a workpiece secured to a movable workpiece holder. For example, Oda et al. U.S. Pat. No. 4,014,495 discloses an automatic welding apparatus wherein the welding torch is secured to a composite frame capable of movement in the longitudinal, lateral and vertical directions as well as being rotatable around a vertical axis. In this particular arrangement, the workpiece holder is adapted to be rotated around a horizontal axis as well as being movable in two mutually orthogonal directions such as the longitudinal and lateral directions. By dividing the necessary degree of freedom of movement between the workpiece holder and the welding tool, it is possible to obtain an orientation of the workpiece with respect to the welding tool which allows downward butt welding and horizontal fillet welding. By way of definition, downward butt welding and horizontal fillet welding are illustrated in FIGS. 7 and 8, respectively, of the present application. However, this approach involves a complex structure as well as demanding intricate computer programming in order to correlate the position of a movable workpiece with respect to a movable welding tool. Additionally, a problem arises when it is desired to perform such downward butt or horizontal fillet welding on the interior of long, box-like or tubular workpieces having narrow entry apertures. To overcome this problem, Oda et al provide a torch fixture with a long arm having a crank or folded portion, as illustrated in FIG. 9 herein, which allows the torch fixture to gain entry to the welding line without interference from the workpiece. By utilizing a longer fixture arm, the accuracy of positioning the torch is reduced and the complexity of the apparatus is increased thereby magnifying maintenance and production considerations.